This invention relates to a receiver which receives a modulated wave which carries a baseband signal of a binary digital signal and which may be subjected to FSK (Frequency Shift Keying), although this invention may not be restricted to the FSK.
Recently, such a receiver has becomes smaller and smaller with the development of a semiconductor integrated circuit technique. In a receiver of a superheterodyne type, a radio section usually comprises a high frequency amplifier, and an intermediate frequency filter, which can not be simply manufactured by the semiconductor integrated circuit technique. Therefore, it has been pointed out that the size of the receiver is limited.
In order to make such a receiver smaller in size and lighter in weight, direct conversion, namely, quadrature detection, has been proposed to detect or demodulate a modulated wave may convey a baseband signal of POCSAC codes. More specifically, a local oscillation frequency is controlled to be matched with a channel frequency received by a receiver and to extract a beat frequency between the local oscillation frequency and the channel frequency. With this receiver, the baseband signal alone is directly extracted from the beat frequency through a low pass filter (LPF) after amplitude limitation and is demodulated by a demodulator into a reproduced baseband signal.
When the direct conversion is used to demodulate the modulated wave, the local oscillation frequency is substantially equal to the channel frequency with only a phase difference component or an offset signal component left between the local oscillation frequency and the channel frequency. Thus, the direct conversion can eliminate render an intermediate frequency and, as a result, can avoid production of an image frequency. Therefore, such direct conversion dispenses with the necessity of a filter which has a high selectivity to attenuate the image frequency which might occur in a high frequency amplifier and an intermediate frequency amplifier.
In addition, the above-mentioned receiver essentially comprises a channel filter for attenuating a jamming wave imposed on adjacent channels. However, it is possible to structure such a channel filter by an active low frequency pass filter which can be manufactured by the semiconductor integrated circuit technique.
Heretofore, a proposal has been made to combine an FSK receiver with a phase locked loop circuit PLL so as to receive a modulated wave which subjects a baseband signal to FSK and which may be referred to as an FSK modulated wave. Specifically, the PLL circuit is used as a local oscillation circuit to produce a local oscillation signal of a local oscillation frequency. The local oscillation frequency is substantially equal to each of channel frequencies. At any rate, the PLL circuit can receive a plurality of channel frequencies by the use of a single quartz crystal in conjunction with a frequency divider and, a read-only memory (ROM).
The FSK receiver may be used as a paging receiver in a paging system which has a plurality of zones to which the channel frequencies are individually preassigned. The above-mentioned receiver which comprises the PLL circuit can receive each of the channel frequencies even when the channel frequencies are changed from one to another due to movement of the receiver from one zone to another zone.
However, the PLL circuit consumes large electric power so as to carry out a PLL operation. In other words, the receiver of the above-mentioned type is disadvantageous in that power consumption becomes large in the PLL circuit. As a result, the paging receiver which is driven by a small battery has a very short life time due to the large power consumption in the PLL circuit.